This invention relates generally to radio frequency antennas and more particularly to multibeam antennas adapted to operate with radio frequency energy having circular polarization.
As is known in the art, an array of antenna elements may be fed through a parallel plate radio frequency lens in such a manner that one or more beams of radio frequency energy are formed. In one known antenna assembly of the type just mentioned and described in U.S. Pat. No. 3,761,936, issued Sept. 25, 1973, entitled "Multibeam Array Antenna," inventors Donald H. Archer, Robert J. Prickett and Curtis P. Hartwig, assigned to the same assignee as the present invention, a linear array of antenna elements, transmission lines, parallel plate radio frequency lens and plurality of feedports are formed on a common substrate using printed circuit techniques. The feedports of the parallel plate radio frequency lens are coupled to the array of antenna elements through different constrained electrical paths, such paths being the printed circuit transmission lines. In another known antenna, described in U.S. Pat. No. 3,754,270, issued Aug. 21, 1973, entitled "Omnidirectional Multibeam Array Antenna," inventor Wilbur H. Thies, Jr., assigned to the same assignee as the present invention, the antenna assembly includes a parallel plate radio frequency lens with feedports formed as printed circuits on a circular dielectric substrate. Antenna elements are coupled to the feedports through different constrained electrical paths, such as through coaxial cables. In either design, with the different constrained electrical paths properly adjusted, it is possible to create any desired number of collimated beams, each one of the beams having a different scan angle. In a copending patent application, Ser. No. 672,701, filed Apr. 1, 1976, inventor George S. Hardie, assigned to the same assignee as the present invention, a multibeam antenna of the type described above, which is useful in applications requiring reduced size, includes a printed circuit parallel plate region having a plurality of feed ports disposed about one portion of the outer periphery of the lens and a continuous, flared radiating structure disposed about a second portion of the parallel plate region, the radiating structure being coupled to the feedports through unconstrained electrical paths provided by the parallel plate region thereby producing substantially collimated beams without requiring different constrained electrical paths between individual antenna elements and the lens. While such antenna is useful in many applications, in applications where such antenna is to be used with radio frequency waves having arbitrary polarization, a separate polarizer is generally required in front of the radio frequency lens and the radiating structure, thereby increasing the size of the antenna.